I guess that:
It is the past, the present and the future.
I reached this by guessing what would reasonably fit in the given text, and noting that:
All the narrative are about the time of when something happens.
When you get to know me
I'll be long time gone.
When you know the past (the one who refers himself/herself as "me"), it is something that do not exists anymore.
If you ever saw me
I was someone else back then.
When you saw the past, it was not the past, it was the present.
I have two younger siblings
and the middle one is your best peer.
The past has two younger siblings, the present and the future. The present is your best peer.
The youngest one you haven't met.
And when you meet,
When you meet the future, he will be the present instead, and immediately become the past.